This invention relates to printing plates that can be made without using a negative. More specifically, it relates to a laser-imageable printing plate. Such plates are particularly useful for flexographic printing, but can be used for offset and lithographic printing.
Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies.
Such plates offer a number of advantages to the printer, based chiefly on their durability and the case with which they can be made. Further improvements, to the degree of resolution (fineness of detail) which can be obtained as well as reductions in cost, would expand the usefulness of these plates. The present invention allows both increased resolution by use of laser processing, and reductions in cost through the elimination of the use of a negative to make the printing plate.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; a protective layer or slip film; and a cover sheet. The backing layer lends support to the plate. It is typically a plastic film or sheet about 5 mils or so thick, which may be transparent or opaque. Polyester films, such as polyethylene terephthalate film, are examples of materials that can be suitably used as the backing. When only a single photocurable layer is present, it may be anywhere from about 25-275 mils thick, and can be formulated from any of a wide variety of known photopolymers, initiators, reactive diluents, fillers, etc. In some plates, there is a second photocurable layer (referred to as an xe2x80x9covercoatxe2x80x9d or xe2x80x9cprintingxe2x80x9d layer) atop this first, base layer of photocurable material. This second layer usually has a similar composition to the first layer, but is generally much thinner, being on the order of less than 10 mils thick. The slip film is a thin (about 0.1-1.0 mils) sheet which is transparent to UV light that protects the photopolymer from dust and increases its ease of handling. The cover sheet is a heavy, protective layer, typically polyester, plastic or paper.
In normal use, the printer will peel the cover sheet off the printing plate, and place a negative on top of the slip film. The plate and negative will then be subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed). Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include use of an air knife or heat plus a blotter.
Exposure of the printing plate is usually carried out by application of a vacuum to ensure good contact between the negative and the plate. Any air gap will cause deterioration of the image. Similarly, any foreign material, such as dirt and dust between the negative and the plate results in loss of image quality.
Even though the slip films are thin and made from transparent materials, they still cause some light scattering and can somewhat limit the resolution which can be obtained from a given image. If the slip film were eliminated, finer and more intricate images could be obtained. Finer resolution would be particularly desirable for the reproduction of elaborate writing as in the case of Japanese characters, and for photographic images.
A negative can be a costly item. For one thing, any negative which is used for printing must be perfect. Any minor flaw will be carried through onto each printed item. As a consequence, effort must be expended to ensure that the negative is precisely made. In addition, the negative is usually made with silver halide compounds which are costly and which are also the source of environmental concerns upon disposal.
Given these considerations, it is clear that any process which would eliminate the use of the negative, or reduce the light scattering effects and other exposure limitations of the slip films, would yield significant advantages in terms of cost, environmental impact, convenience, and image quality over the present methods.
The inventor has found a way to obtain these advantages by using a laser that is guided by an image stored in an electronic data file to create an in situ negative on a modified slip film, and then exposing and developing the printing plate in the usual manner. As a result, the printer need not rely on the use of negatives and all their supporting equipment, and can rely instead on a scanned and stored image. Such images can be readily altered for different purposes, thus adding to the printer""s convenience and flexibility. In addition, this method is compatible with the current developing and printing equipment, so expensive alterations to the other equipment are not required.
Laser engraving of various materials, such as wood and metal, is well known. Laser engraving of cured hard rubber or lithographic plates is also known. If this procedure were applied to a flexographic printing plate, the plate would first be exposed to UV light without an image. Then the laser would be used to engrave an image on the hardened plate. This has been attempted, but found to be too slow to be commercially competitive. Flexographic printing plates require high relief (generally, 30-40 mil high letters) which take a long time to engrave.
Direct exposure of a photopolymer using a laser is also known. This procedure uses a precisely guided laser to replace the UV flood lamps which are normally used to expose the plate. U.S. Pat. No. 4,248,959, issued to Jeffers et al. Feb. 3, 1981, relates to the direct exposure of a photosensitive polymer plate using a laser guided by a computer-generated image. The disclosed method is not suitable for the development of flexographic printing plates, because the thickness of such plates hampers the cure. Again, the process is too slow to be commercially competitive.
Other efforts have focussed on generating an image directly in contact with a photocurable layer. U.S. Pat. No. 5,015,553 issued to Grandmont et al. May 14, 1991 relates to a method of making a UV photoresist for a printed circuit board, using a computer-assisted design (CAD) driven photoplotter which selectively exposes a photographic imaging layer without affecting the underlying UV sensitive photoresist. The image layer is then chemically developed on the board and used as an in situ mask for the underlying UV resist during exposure to UV light. After the exposure, the image layer is peeled off to allow conventional processing of the resist. The process requires at least two development steps for the entire plate, and also requires the use of a peelable cover sheet interposed between the image layer and the photocurable layer.
Laser ablation of polymers from relatively insensitive substrates is known. U.S. Pat. No. 4,020,762 issued to Peterson May 3, 1977 relates to a method of making a sensitized aluminum printing plate for offset lithography. An aluminum sheet was coated with a mixture of finely divided carbon, nitrocellulose, a non-oxidizing alkyd resin, a diazo sensitizer, cellulose acetate, butylacetate, xylene and ethyl cellosolve. The coating was at least partially etched with a YAG laser. It is not clear whether all the coating was removed from the aluminum substrate although the text alludes to this result. The patentee discloses that the etched areas became sensitive to UV light, and that the etched areas, after exposure to UV light and development, accepted ink, while the areas which were not etched accepted water. No quantitative results arc presented. There is no indication that the liquid coating in the reference would be usable as a flexographic printing plate. There is no indication that the laser ablation was precise enough to allow removal of a polymer layer to uncover a photosensitive polymer layer directly beneath.
Laser ablation of polymers from sensitive substrates is also known. U.S. Pat. No. 5,925,500 issued to Yang, et al. Jul. 20, 1999 relates to a method of making a laser imaged printing plate utilizing an ultraviolet absorbing layer. Here, the negative was replaced by a slip-film doped with a UV absorber. The doped slip film was laminated to a photopolymer layer and was ablated from the photopolymer layer using a laser operating at a wavelength in the UV region, i.e., 300-400 nm. The resulting imaged UV absorber-doped layer effectively becomes the negative and can be subjected to typical UV flood exposure and development. Here, however, the disclosed method is not commercially practical because practicing the invention is limited to photo-ablation with a laser operating in the UV region. This type of laser is expensive, difficult to maintain, and the optics used in such lasers are unstable and easily damaged.
Lasers have also been used to physically transfer small amounts of polymer from one layer of a multilayer article to another. U.S. Pat. No. 5,156,938 issued to Foley et al. Oct. 30, 1992, relates to a method of laser-induced ablative transfer imaging suitable for the production of masks (negatives) for the graphic arts and printed circuit industries. In this process, a laser-sensitive material is physically displaced from a donor layer of a multilayer structure to a receptor layer. This is described as an ablative transfer because some of the materials from the donor layer arc ablated while other materials are deposited on the receptor layer.
The inventors have discovered that if a multilayer slip film is constructed comprising at least two layers wherein at least one layer comprises a strong UV absorber, and wherein at least one other layer comprises an IR absorber having high absorptivity, a more stable IR laser that is readily available such as, for example, Nd-YAG lasers operating at 1064 nm, and diode array lasers operating at 830 nm, can be used to engrave the slip film. The IR absorber layer enables the slip film coating to ablate cleaner and easier while the UV absorber layer allows the slip film to effectively become a negative that is created in situ. There is no need to separately manufacture a negative, or to eventually dispose of silver halide. Also, the light scattering effects resulting from the presence of a separate conventional slip film underlying the negative are eliminated, thereby increasing resolution of the image.
According to one embodiment, the present invention provides a laser-imageable and photocurable article comprising a backing layer, a photocurable layer disposed upon said backing, having a low absorptivity of radiation at a selected wavelength in the range of 300-400 nm and an initiator activatable at the selected wavelength, and a multilayer photoablative slip film. The layers of the multilayer slip film comprise a film-forming polymer and a self-oxidizing binder. In particular, at least one of the layers of the multilayer photoablative slip film comprises a UV absorber having high absorptivity in the wavelength range of 300-400 nm, and at least one other layer of the multilayer photoablative slip film comprises an IR absorber having high absorptivity at either the 830 nm wavelength or the 1064 nm wavelength. The multi layer photoablative slip film is capable of being photoablated by a laser operating at an energy level in the IR or near-IR wavelengths, and wherein unablated areas of the multilayer slip film are capable of absorbing substantially all irradiated light from an ultra-violet light source, such that areas of the photocurable layer under ablated areas of the multilayer photoablative slip film are cured, and areas of the photocurable layer under unablated areas of the multilayer photoablative slip film remain uncured, upon exposure of the article to an actinic radiation light source.